Field of the Invention
The present invention relates to an electric power steering circuit device which drives and controls a motor by a bridge circuit to generate assisting steering torque for assisting a steering motion of the steering wheel of a vehicle in accordance with the speed of the vehicle, and more particularly to an electric power steering circuit device in which a support member integral with a connector is integrally formed with wiring patterns to achieve device miniaturization and reduce production cost.
Description of the Related Art
FIG. 3 is a circuit diagram showing a general electric power steering circuit device. In the figure, a motor 40 for outputting an assisting torque to the steering wheel (not shown) of a vehicle is supplied with a motor current IM by a battery 41 and driven.
The ripple component of the motor current IM is absorbed by a large capacity (about 3600 .mu.F) capacitor 42 and is detected by a shunt resistor 43. Also, the motor current IM is switched according to the magnitude and direction of an assisting torque by a bridge circuit 44 comprising a plurality of semiconductor switching elements (for example, FETs) Q1 to Q4.
The capacitor 42 is connected at its one end to ground through a conductive line L1. The semiconductor switching elements Q1 to Q4 are bridge-connected by wiring patterns P1 and P2 and constitute the bridge circuit 44. Also, these wiring patterns P1 and P2 connect this bridge circuit 44 to the shunt resistor 43. The output terminal of the bridge circuit 44 is constituted by a wiring pattern P3.
The motor 40 and the battery 41 are connected to the bridge circuit 44 through a connector 45 comprising a plurality of lead terminals. The motor 40, the battery 41 and the connector 45 are connected through an external wiring L2. The motor current IM is interrupted as needed by a normally open relay 46. The relay 46, the capacitor 42, and the shunt resistor 43 are interconnected through a wiring pattern P4. The connector 45 is connected to ground through a wiring pattern P5. The wiring pattern P3 which becomes the output of the bridge circuit 44 is connected to the connector 45.
The motor 40 is driven through the bridge 44 by a drive circuit 47. Also, this drive circuit 47 drives the relay 46. The drive circuit 47 is connected through a conductive line L3 to the exciting coil of the relay 46 and through a conductive line L4 to the bridge circuit 44. The motor current IM is detected by motor current detection means 48 through the opposite ends of the shunt resistor 43. The drive circuit 47 and the motor current detection means 48 constitute the peripheral circuit elements of a microcomputer to be described later.
The steering torque T of the steering wheel is detected by a torque sensor 50, and the speed or velocity V of the vehicle is detected by a speed or velocity sensor 51.
The microcomputer 55 (ECU) calculates an assisting torque based on the steering torque T and the vehicle speed V, and generates a drive signal corresponding to the assisting torque, based on the motor current IM fed back thereto. A rotational direction command Do and a current control quantity Io for controlling the bridge circuit 44 are output as drive signals to the drive circuit 47.
The microcomputer 55 comprises motor current decision means 56 for generating a motor current command Im corresponding to the rotational direction command Do and assisting torque of the motor 40, subtraction means 57 for outputting a current deviation .DELTA.I between the motor current command Im and the motor current IM, and PID calculation means 58 for calculating correction quantities of P (proportion), I (integration), and D (differentiation) terms from the current deviation .DELTA.I to generate a current control quantity Io corresponding to a PWM duty ratio.
Also, the microcomputer 55 includes an AD converter and a PWM timer, and it has a known self-diagnosis function, although they are not shown. The microcomputer 55 always performs a self-diagnosis if the system operates normally, and if an abnormality occurs, will open the relay 46 through the drive circuit 47 to interrupt the motor current IM. The microcomputer 55 is connected to the drive circuit 47 through a conductive line L5.
In general, the circuit elements 42 to 44, wiring patterns P1 to P5, and conductive lines L1 and L2 interposed between the motor 40 and the battery 41 are of increased size so that they can correspond to the motor current IM of a large current, taking in consideration of their heat-radiating properties (heat-resisting properties) and durability, as will be described later. On the other hand, the microcomputer 55, peripheral circuit elements including the drive circuit 47 and the motor current detection circuit 48, and the conductive lines L3 to L5 are reduced in size so that they can correspond to small currents and meet higher density requirements.
FIG. 4 is a plan view showing the structure of a conventional electric power steering circuit device, and Q1 to Q4, 42 to 45, and 55 are the same as those shown in FIG. 3. In FIG. 4, each of the semiconductor switching elements Q1 to Q4 is constituted by a pair of field-effect transistors (FETs) covered with resin, the large capacity capacitor 42 is constituted by three capacitors, and the microcomputer 55 is constituted by a one-chip integrated circuit. Also, peripheral circuit elements, wiring patterns, and conductive lines are omitted to reduce the complexity of illustration, and only representative structural elements are shown.
An insulated printed-circuit board 2 is mounted on the bottom surface of a box-shaped metal frame 1 which functions both as a shielding plate and a heat radiating plate. On the inner side face of the metal frame 1 there is joined one end face of each of heat radiating plates 3 made of, for example, aluminum. The circuit elements 42 to 45 and 55 are mounted on the insulated printed-circuit board 2. The semiconductor switching elements Q1 to Q4 are joined to the other end faces of the heat radiating plates 3.
Conductive plates 4a to 4e correspond to the wiring patterns P1 to P5, respectively. For these conductive plates, conductive plates wider and thicker than other wiring patterns on the insulated printed-circuit board 2 are used to exclusively correspond to a large current.
Next, the operation of the conventional electric power steering circuit device shown in FIG. 4 will be described while referring to FIG. 3.
The steering torque T detected by the torque sensor 50 and the speed V detected by the speed sensor 51 are input to the microcomputer 55, and also the motor current IM from the shunt resistor 43 is fed back to the microcomputer 55. Based these inputs, the microcomputer 55 generates a rotational direction command Do for the power steering and a current control quantity Io corresponding to an assisting torque quantity, and outputs them to the drive circuit 47 through the conductive line L5.
The drive circuit 47 closes, in its steady drive state, the normally open relay 46 by its command through the conductive line L3. If a rotational direction command Do and a current control quantity Io are input, the drive circuit 47 will generate a PWM drive signal and apply it to each of the semiconductor switching elements Q1 to Q4 of the bridge circuit 44 through the conductive line L4.
With this, the motor current IM is supplied from the battery 41 to the motor 40 through the external wiring L2, the connector 45, the relay 46, the wiring pattern P4, the shunt resistor 43, the wiring pattern P1, the bridge circuit 44, the wiring pattern P3, the connector 45, and the external wiring L2. The motor 40 is driven by this motor current IM and outputs a predetermined quantity of assisting torque in a predetermined direction.
At this time, the motor current IM is detected via the shunt resistor 43 and the motor current detection means 48 and fed back to the subtraction means 57 of the microcomputer 55 so that it matches with the motor current command Im output by the motor current decision means 56. Also, the motor current IM includes the ripple component caused by the switching operation as the bridge circuit 44 is PWM-driven, but it is smoothed and controlled by the large capacity capacitor 42.
However, the value of the motor current IM controlled by this kind of electric power steering circuit device is about 25 A even for a sub-compact automobile and reaches about 60 to 80 A for a compact automobile. Therefore, the semiconductor switching elements Q1 to Q4 constituting the bridge circuit 44 need to be increased in size according to the magnitude of the motor current IM, and as shown, a plurality of semiconductor switching elements need to be connected in parallel so that the generation of heat while they are on or PWM-switched can be reduced.
Also, the above-described heat radiating plates 3 are needed in order to radiate the heat generated by the semiconductor switching elements Q1 to Q4. As the motor current IM increases, the semiconductor switching elements Q1 to Q4 must be increased in number and at the same time the heat radiating plates 3 must be increased in size.
Further, the wiring patterns P1, P2 and P4 leading from the terminals of the connector 45 through the relay 46, the shunt resistor 43, and the bridge circuit 44 to ground and the wiring pattern P3 leading from the bridge circuit 44 to the motor 40 become longer physically in proportion to increases in the motor current IM, the number of the semiconductor switching elements Q1 to Q4, and the size of the heat radiating plates 3.
Consequently, if a rise in temperature becomes greater because of the generation of heat caused by a voltage drop on each of the wiring patterns P1 to P5, there is the possibility that the heat-resisting properties and durability of the wiring patterns P1 to P5 will be deteriorated. Therefore, in the conventional electric power steering circuit device, in order to prevent this, the wiring plates 4a to 4e with large widths and thicknesses are exclusively used for a large current as shown in FIG. 4. Therefore, this causes an increase in size of the insulated printed-circuit board 2.
Also, the capacitor 42, the shunt resistor 43, and the relay 46 are increased in size as the motor current IM becomes greater, and if these are mounted on the insulated printed-circuit board 2, an increase in the mounting space will cause an increase in size of the insulated printed-circuit board 2.
As described above, in the conventional electric power steering circuit device, the capacitor 42, shunt resistor 43, bridge circuit 44, heat radiating plate 3, and wiring plates 4a to 4e (wiring patterns P1 to P5) which correspond to a large current have been mounted on the insulated printed-circuit board 2, so the insulated printed-circuit board 2 is also increased in size as the circuit elements 42 to 44 and the wiring patterns P1 to P5 are increased in size. As a result, there is the problem that the device weight is increased, the cost is increased, and the ability to mount elements on the device is deteriorated.